Gunmen mounted what appeared to be a coup attempt when they stormed Haiti's presidential palace today.

But, a few hours later, they fled the building in the capital Port-au-Prince, killing two bystanders as they sped away in pick-up trucks.

President Jean-Bertrand Aristide and his wife were at their home about three miles away when the palace came under attack, said National Palace spokesman Jacques Maurice.

Hundreds of his supporters, wielding machetes, surrounded the palace, shouting, ''We'll never accept another coup d'etat.''

Aristide was first elected president in 1990 and stayed in power only eight months before the army ousted him in a coup in September 1991. He started his second term in February.

In apparent retribution for the palace attack, Aristide supporters torched the headquarters of opposition parties.

Government officials described the attack as an attempted coup.

Before attacking the national palace, the gunmen attempted to assault the national penitentiary, Maurice said. When they were rebuffed, they then went on to the palace.

The gunmen lobbed a grenade at the National Palace about 2 am and then began firing as they entered. They killed two police officers guarding the palace, Maurice said.

The presidential mansion is protected by hundreds of guards, and it was unclear how the gunmen penetrated the security.

''This is an attempted coup d'etat,'' Maurice said. ''This is not a staged event.''

A government official said the attackers identified their leader as the former Haitian town's police chief, Guy Philippe, who fled to the Dominican Republic earlier this year.

Since Aristide's Lavalas Family party swept parliamentary and local elections in May 2000, the Caribbean nation has been mired in unrest with the main opposition group calling the elections fraudulent and foreign donors refusing to release desperately needed aid until results are revised.

There has also been mounting grass-roots opposition to Aristide within his own party. Protesters have accused Aristide of failing to deliver on promises of basic services.